Adjusting an amount of coherence of a light beam

ABSTRACT

Techniques for controlling an optical system include accessing a measured value of a property of a particular pulse of a pulsed light beam emitted from the optical system, the property being related to an amount of coherence of the light beam; comparing the measured value of the property of the light beam to a target value of the property; determining whether to generate a control signal based on the comparison; and if a control signal is generated based on the comparison, adjusting the amount of coherence in the light beam by modifying an aspect of the optical system based on the control signal to reduce an amount of coherence of a pulse that is subsequent to the particular pulse.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/256,196, filed Sep. 2, 2016, now allowed, and titled ADJUSTING ANAMOUNT OF COHERENCE OF A LIGHT BEAM, which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

This disclosure relates to adjusting an amount of coherence of a beam oflight. For example, the techniques discussed below may be used to reduceor suppress coherence in a pulsed optical beam.

BACKGROUND

Photolithography is the process by which semiconductor circuitry ispatterned on a substrate such as a silicon wafer. A photolithographyoptical source provides the deep ultraviolet (DUV) light used to exposea photoresist on the wafer. DUV light for photolithography is generatedby excimer optical sources. Often, the optical source is a laser sourceand the pulsed light beam is a pulsed laser beam. The light beam ispassed through a beam delivery unit, a reticle or a mask, and thenprojected onto a prepared silicon wafer. In this way, a chip design ispatterned onto a photoresist that is then etched and cleaned, and thenthe process repeats.

SUMMARY

In one general aspect, a method of controlling an optical systemincludes accessing a measured value of a property of a particular pulseof a pulsed light beam emitted from the optical system, the propertybeing related to an amount of coherence of the light beam; comparing themeasured value of the property of the light beam to a target value ofthe property; determining whether to generate a control signal based onthe comparison; and if a control signal is generated based on thecomparison, adjusting the amount of coherence in the light beam bymodifying an aspect of the optical system based on the control signal toreduce an amount of coherence of a pulse that is subsequent to theparticular pulse.

Implementations may include one or more of the following features. Theproperty of the light beam may include a divergence of the light beam.The divergence of the light beam may be measured outside of the opticalsystem.

The pulse that is subsequent to the particular pulse may be the pulsethat immediately follows the particular pulse.

Adjusting the amount of coherence in the light beam by modifying thevalue of the property of the light beam based on the control signal mayinclude applying the control signal to an optical element of the opticalsystem, the optical element including an optical surface that interactswith the light beam in the optical system to at least partiallydetermine the value of the property of the light beam emitted from theoptical system, the application of the control signal being sufficientto move the optical surface. The application of the control signal beingsufficient to move the optical surface may include the application ofthe control signal being sufficient to change the shape of the opticalsurface, change the position of the optical surface, change the angle ofthe optical surface relative to a direction of propagation of the pulsedlight beam, or change a curvature of the optical surface. The opticalsurface may include a plurality of portions, and application of thecontrol signal is sufficient to move at least one of the plurality ofportions of the optical surface relative to at least one of the otherportions.

In some implementations, an operating condition of the optical system isdetermined, the operating condition being associated with one or moreoperating parameters, and if a control signal is not generated based onthe comparison, the measured value of the property of the light beam maybe declared as a desired value of the property for the determinedoperating condition. Declaring the measured value of the property of thelight beam as a desired value of the property for the determinedoperating condition may include storing the measured value inassociation with the operating parameters of the operating condition.The operating parameters may include one or more of a pressure of a gainmedium of the optical system, a temperature of a gain medium of theoptical system, a repetition rate of the light beam emitted from theoptical system, and an amount of force applied to an optical elementincluding an optical surface that interacts with the light beam in theoptical system to at least partially determine the value of the propertyof the light beam emitted from the optical system. An indication of achange in the operating condition of the optical system to a currentoperating condition may be received, the desired value of the propertyfor the current operating condition may be determined from the storeddata; and the target value may be set to the determined desired value.The indication of a change in the operating condition of the opticalsystem may include an indication of a change in the repetition rate ofthe light beam emitted from the optical system to a new repetition rate,and the indication of the change in the repetition rate may be receivedfrom a lithography exposure apparatus that receives the light beam fromthe optical system.

In some implementations, accessing a measured value of a property of alight beam includes measuring the value of the property of the lightbeam at a first time and at a second time, the second time occurringafter the first time and after adjusting the amount of coherence in thelight beam. The value of the property of the light beam at the secondtime may be compared to the target value, and whether to generate asecond control signal may be determined based on the comparison.

In another general aspect, a method of controlling an optical systemincludes obtaining a value of a property of an optical beam emitted froman optical system at a plurality of operating conditions, the obtainedvalue of the property being associated with an amount of coherence inthe optical beam, and each of the operating conditions being associatedwith one or more operating parameters and status information related toan optical element, the optical element including an optical surfacethat interacts with the light beam to at least partially determine theobtained value of the property of the light beam; storing the obtainedvalue of the property and the status information related to the opticalelement for each operating condition; operating the optical system at acurrent operating condition, the current operating condition beingassociated with current values of one or more of the operatingparameters and a current status information related to the opticalelement; receiving an indication of a change in the current operatingcondition of the optical system, the indication of a change including anew value of one or more of the operating parameters; determining statusinformation related to the optical element associated with the new valuefrom the stored data; and applying a command signal based on thedetermined status information to the optical element.

Implementations may include one or more of the following features. Theobtained value of the property of the optical beam may be associatedwith an acceptable amount of coherence in the optical beam for each ofthe plurality of operating conditions, and another value of the propertyof the light beam may be obtained by measuring the value of the propertyof the light beam after applying the determined status information tothe optical element; the another value may be compared to the obtainedvalue of the property associated with the new value of the one or moreoperating parameters; and whether the another value of the property isacceptable may be determined based on the comparison.

The status information and the determined status information may includean amount of force applied to the optical element, and, if the anothervalue of the property is not acceptable, a third amount of force may bedetermined based on the new force and a pre-determined constant.

The status information related to the optical element may include aposition of the surface of the optical element.

The status information related to the optical element may include anamount of force applied to the optical element, the force beingsufficient to determine a position of the surface of the opticalelement.

In another general aspect, a system for a light source includes anoptical coupler; a first optical system including a first opticalelement; a second optical system including a second optical element; anda gain medium between the first optical system and the second opticalsystem, where the first optical element, the second optical element, andthe optical coupler define a closed path, at least a portion of theclosed path passing through the gain medium, and one or more of thefirst optical element and the second optical element include anoptically reflective surface positioned to interact with a light beamthat propagates on the closed path, the surface including a plurality ofportions, at least one of the portions having a surface profile that isdifferent from the surface profiles of the other segments.

Implementations may include one or more of the following features. Thefirst optical element may include the surface with the plurality ofportions, at least one of the portions may be a mirror and at least oneof the portions may be a grating. The portions may be adjacent to eachother, and a flexible member may be between any two adjacent portionssuch that at least one portion is movable relative to an adjacentportion. In some implementations, any portion is movable relative to anadjacent portion. The surface profile of a portion may be determined byone or more of a radius of curvature and a surface characteristic of theportion.

In another general aspect, a system for a light source includes anoptical coupler; a first optical system including a first reflectiveoptical element; a second optical system including a second reflectiveoptical element; and a gain medium between the first optical system andthe second optical system, where the first reflective optical element,the second reflective optical element, and the optical coupler define aclosed path, at least a portion of the closed path passing through thegain medium, the second reflective optical element includes a pluralityof distinct reflective surfaces, a first of the distinct reflectivesurfaces positioned to receive a light beam from the gain medium and toreflect the light beam to a second of the distinct reflective surfaces,the second of the distinct reflective surfaces being positioned toreceive the light beam from the first distinct reflective surface and toreflect the light beam into the gain medium, and at least one of thedistinct reflective surfaces being one or more of physically separatedfrom the other reflective surfaces and movable relative to the otherreflective surfaces.

At least one of the distinct reflective surfaces of the second opticalelement may be movable relative to the other reflective surfaces, andthe second optical system also may include an actuator coupled to the atleast one movable reflective surface, the actuator being configured tocause the movable reflective surface to move in response to a commandsignal.

In another general aspect, a system includes an optical systemconfigured to amplify a pulsed optical beam, the pulsed optical beamhaving a repetition rate, the optical system including an opticalcoupler; a first optical element; a gain medium; and a second opticalelement, the gain medium being between the first optical element and thesecond optical element, and the first optical element, the secondoptical element, and the optical coupler defining a closed path thatpasses through the gain medium; a lithography exposure apparatusconfigured to receive the amplified pulsed optical beam; and a controlsystem coupled to the optical system and the lithography exposureapparatus, the control system configured to: receive a request to changethe repetition rate of the pulsed optical beam from a first repetitionrate to a second repetition rate, determine a value of property of thepulsed optical beam at the second repetition rate, determine whether thevalue of the property is acceptable, and if the value of the property isnot acceptable, generate a command signal based on the determined valueof the property, and provide the command signal to the optical system,the command signal being sufficient to physically modify a controllableoptical element that interacts with light that propagates on the closedpath of the optical system.

Implementations may include one or more of the following features. Thevalue of the property of the pulsed optical beam may be a measure ofcoherence of one or more pulses of the pulsed optical beam.

The control system may be further configured to determine whether thevalue of the property is acceptable after providing the command signalto the optical system.

Determining a value of the property of the pulsed optical beam at thesecond repetition rate may include accessing a value of an amount offorce applied to the controllable optical element, and providing thecommand signal to the controllable optical element may include applyinga different amount of force to the controllable optical element.

Determining a value of the property of the pulsed optical beam at thesecond repetition rate may include measuring a divergence of the pulsedoptical beam when the optical system operates at the second repetitionrate.

Determining a value of the property of the pulsed optical beam at thesecond repetition rate may include accessing a value of the property ofthe pulsed optical beam stored in an electronic storage. The value ofthe property of the pulsed optical beam stored in an electronic storagemay be a value measured previously and while the optical system operatedat the second repetition rate.

The request to change the repetition rate may be received from thelithography exposure apparatus.

The control system also may include an operator interface, the operatorinterface being configured to allow operator interaction with thelithography exposure apparatus, and the request to change the repetitionrate may be received from the operator interface.

The optical system also may include electrodes configured to excite thegain medium, and the repetition rate of the pulsed optical beam may becontrollable by controlling a firing pattern of the electrodes, thefiring pattern specifying when the electrodes are in an active statethat is sufficient to excite the gain medium.

The controllable optical element may be one or more of the first opticalelement and the second optical element.

Implementations of any of the techniques described above and herein mayinclude a process, an apparatus, a control system, instructions storedon a non-transient machine-readable computer medium, and/or a method.The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features will beapparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of an example of a photolithography system.

FIG. 1B is a block diagram of an example of a mask used in thephotolithography system of FIG. 1A.

FIG. 2 is a block diagram of another exemplary photolithography system.

FIGS. 3A, 3B, and 3C are graphs of examples of signals that are used tocontrol an optical source that is part of a photolithography system.

FIGS. 4 and 5A are block diagrams of another example of aphotolithography system.

FIG. 5B is a side view in the x-y plane of a resonator cavity of thesystems of FIGS. 4 and 5A.

FIG. 5C is a side view in the y-z plane of the resonator cavity of FIG.5B.

FIGS. 6-9 are side views of exemplary reflective optical elements.

FIGS. 10-13 are side views of exemplary beam turning optical systems.

FIGS. 14 and 15 are flow charts of exemplary processes for controllingan optical system.

DETAILED DESCRIPTION

Referring to FIG. 1A, a photolithography system 100 includes an optical(or light) source 105 that provides a light beam 160 to lithographyexposure apparatus 115, which processes a wafer 120. The light beam 160is a pulsed light beam that includes pulses of light separated from eachother in time. The lithography exposure apparatus 115 includes aprojection optical system 125 through which the light beam 160 passesprior to reaching the wafer 120, and a detector 122. The detector 122may be, for example, a camera or other device that is able to capture animage of the wafer 120 or the light beam 160 at the wafer 120, or anoptical detector that is able to capture data that describescharacteristics of the beam 160, such as intensity of the beam 160 atthe wafer 120 in the x-y plane. The lithography exposure apparatus 115can be a liquid immersion system or a dry system. The photolithographysystem 100 also may include a control system 150 to control the lightsource 105. In implementations that include the control system 150, thecontrol system 150 is coupled to the light source 105 and also may becoupled to the lithography exposure apparatus 115.

Microelectronic features are formed on the wafer 120 by, for example,exposing a layer of radiation-sensitive photoresist material on thewafer 120 with the light beam 160. Referring also to FIG. 1B, theprojection optical system 125 includes a slit 126, a mask 124, and aprojection lens 127. After reaching the projection optical system 125,the light beam 160 passes through the slit 126. In the example of FIGS.1A and 1B, the slit 126 is rectangular and shapes the light beam 160into an elongated rectangular shaped light beam. This shaped light beamthen passes through the mask 124. A pattern is formed on the mask 124,and the pattern determines which portions of the shaped light beam aretransmitted by the mask 124 and which are blocked by the mask 124. Thedesign of the pattern is determined by the specific microelectroniccircuit design that is to be formed on the wafer 120. The portions ofthe shaped light beam that are transmitted by the mask 124 pass through(and can be focused by) the projection lens 127 and expose the wafer120.

The critical dimension (CD), which is the smallest feature size that canbe printed on the wafer 120 by the system 100, depends on the wavelengthof the light beam 160. The optical source 105 produces light having awavelength that is nominally at a center wavelength. To maintain auniform CD of the microelectronic features printed on the wafer 120, andon other wafers exposed by the system 100, the center wavelength of thelight beam 160 should remain at an expected or target center wavelengthor within a range of wavelengths around the target wavelength. Thus, inaddition to maintaining the center wavelength at the target centerwavelength, it is desirable to make the bandwidth of the light beam 160(the range of wavelengths in the light beam 160) small or narrow. Avariety of techniques may be used to decrease the bandwidth of the lightbeam 160. For example, the light beam 160 may be narrowed throughinteraction with a dispersive optical element, such as a grating, thatremoves some wavelengths from the light beam 160 while retaining others.However, such bandwidth-reducing techniques may increase the coherenceof the light beam 160. Increasing the coherence may lead to speckle, thepresence of which may cause an unwanted variation in the CD.

The light beam 160 is an electromagnetic field and may exhibit spatialand/or temporal coherence. The beam 160 has spatial coherence when thephase of the electromagnetic field at different locations is the same.The beam 160 has temporal coherence when the phase of theelectromagnetic field at a single location is the same at differenttimes. When the beam 160 has spatial and/or temporal coherence, thewavefronts in the beam 160 may randomly interfere with each other toproduce speckle. The speckle causes spatial and/or temporal noise in thelight beam 160, and the speckle may cause a speckle pattern that has anon-uniform intensity profile in the x-y plane at the optical system 125and/or the wafer 120.

Thus, when the beam 160 is highly coherent, speckle is present in thebeam 160, and the light delivered to the projection optical system 125and/or the wafer 120 includes noise. For example, the light at the wafer120, which is delivered by the beam 160, may have a non-uniformintensity in the x-y plane, and this non-uniformity can result in unevenexposure of the photoresist and a variation in the CD. For example,speckle may result in variation of the sizes of the exposed regions ofphotoresist that make up the microelectronic features, causing thefeatures to be improperly formed and defective.

The techniques discussed below reduce the coherence of the light beam160, thereby reducing the amount of speckle, without increasing thebandwidth of the light beam 160. As discussed in greater detail below,the coherence of the light beam 160 is reduced by changing the modalcontent of the light beam 160. The modal content may be changed bymodifying a resonator cavity of the optical source 105, for example,through manipulation or repositioning of one or more optical elementsthat define the resonator cavity. An example of a resonator cavity isdiscussed below with respect to FIGS. 2, 4, 5A, 5B, and 5C. FIGS. 6-13show optical elements that include one or more optical elements that areadjustable in a manner that allows the coherence of the light beam 160to be changed. The modifications to the resonator cavity may be made atthe time of manufacture, performed by a human operator in the field, orcontrolled in a closed-loop manner with the control system 150 while thelight source 105 operates and produces light. In some implementations(such as the example discussed with respect to FIGS. 5, 14, and 15), thecoherence of the light beam 160 may be changed in response to a commandfrom the lithography exposure apparatus 115.

In some implementations, a characteristic that relates to the coherenceof the light beam 160, such as, for example, a divergence of the lightbeam 160, is measured to determine whether the amount of coherence inthe light beam 160 should be adjusted. Divergence of an electromagneticbeam (such as the light beam 160) is an angular measure of the increasein beam size as the beam propagates away from an aperture in a planeparallel to the direction of propagation. Thus, the divergence of thebeam 160 may be based on an increase in the diameter of the beam 160 ina plane perpendicular to the direction of propagation. For example, inimplementations in which the light beam 160 has a circular cross-sectionin a plane that is perpendicular to the direction of propagation, thedivergence of the beam 160 may be based on an increase in the radius ofthe beam 160. In some implementations, the light beam 160 may have arectangular cross-section, and the divergence of the beam may be basedon an increase in either or both of the vertical and horizontaldirections. The divergence of an electromagnetic wave (such as the beam160) increases as the coherence of the wave decreases. Examples ofimplementations in which a property of the beam 160 may be measured todetermine whether the coherence of the beam 160 should be reduced arediscussed with respect to FIGS. 5, 14, and 15.

Before discussing the details of the techniques for reducing coherenceof the light beam 160, example implementations of the light source 105are discussed with respect to FIGS. 2 and 3A-3C.

Referring also to FIG. 2, a block diagram of an exemplaryphotolithography system 200 is shown. In the photolithography system200, an exemplary optical source 205 is used as the optical source 105(FIG. 1). The optical source 205 produces a pulsed light beam 260, whichis provided to the lithography exposure apparatus 115. The opticalsource 205 can be, for example, an excimer optical source that outputsthe pulsed light beam 260 (which can be a laser beam). As the pulsedlight beam 260 enters the lithography exposure apparatus 115, it isdirected through the projection optical system 125 and projected ontothe wafer 120. In this way, one or more microelectronic features arepatterned onto a photoresist on the wafer 120 that is then developed andcleaned prior to subsequent process steps, and the process repeats. Thephotolithography system 200 also includes the control system 150, which,in the example of FIG. 2, is connected to components of the opticalsource 205 as well as to the lithography exposure apparatus 115 tocontrol various operations of the system 200.

In the example shown in FIG. 2, the optical source 205 is a two-stagelaser system that includes a master oscillator (MO) 212 that provides aseed light beam 224 to a power amplifier (PA) 230. The MO 212 and the PA230 may be considered to be subsystems of the optical source 205 orsystems that are part of the optical source 205. The power amplifier 230receives the seed light beam 224 from the master oscillator 212 andamplifies the seed light beam 224 to generate the light beam 260 for usein the lithography exposure apparatus 115. For example, the masteroscillator 212 can emit a pulsed seed light beam, with seed pulseenergies of approximately 1 milliJoule (mJ) per pulse, and these seedpulses can be amplified by the power amplifier 230 to about 10 to 15 mJ.

The master oscillator 212 includes a discharge chamber 240 having twoelongated electrodes 217, a gain medium 219 that is a gas mixture, and afan for circulating gas between the electrodes 217. A resonator isformed between a line narrowing module 216 on one side of the dischargechamber 240 and an output coupler 218 on a second side of the dischargechamber 240. The line narrowing module 216 can include a diffractiveoptic such as a grating that finely tunes the spectral output of thedischarge chamber 240. The master oscillator 212 also includes a linecenter analysis module 220 that receives an output light beam from theoutput coupler 218 and a beam coupling optical system 222 that modifiesthe size or shape of the output light beam as needed to form the seedlight beam 224. The line center analysis module 220 is a measurementsystem that can be used to measure or monitor the wavelength of the seedlight beam 224. The line center analysis module 220 can be placed atother locations in the optical source 205, or it can be placed at theoutput of the optical source 205.

The gas mixture used in the discharge chamber 240 can be any gassuitable for producing a light beam at the wavelength and bandwidthrequired for the application. For an excimer source, the gas mixture maycontain a noble gas (rare gas) such as, for example, argon or krypton, ahalogen, such as, for example, fluorine or chlorine and traces of xenonapart from helium and/or neon as buffer gas. Specific examples of thegas mixture include argon fluoride (ArF), which emits light at awavelength of about 193 nm, krypton fluoride (KrF), which emits light ata wavelength of about 248 nm, or xenon chloride (XeCl), which emitslight at a wavelength of about 351 nm. The excimer gain medium (the gasmixture) is pumped with short (for example, nanosecond) current pulsesin a high-voltage electric discharge by application of a voltage to theelongated electrodes 217.

The power amplifier 230 includes a beam coupling optical system 232 thatreceives the seed light beam 224 from the master oscillator 212 anddirects the light beam through a discharge chamber 240, and to a beamturning optical element 252, which modifies or changes the direction ofthe seed light beam 224 so that it is sent back into the dischargechamber 240. The discharge chamber 240 includes a pair of elongatedelectrodes 241, a gain medium 219 that is a gas mixture, and a fan forcirculating the gas mixture between the electrodes 241.

The output light beam 260 is directed through a bandwidth analysismodule 262, where various parameters (such as the bandwidth or thewavelength) of the beam 260 can be measured. The output light beam 260can also be directed through a pulse stretcher, where each of the pulsesof the output light beam 260 is stretched in time, for example, in anoptical delay unit, to adjust for performance properties of the lightbeam that impinges the lithography exposure apparatus 115.

The control system 150 may be connected to various components of theoptical source 205. For example, the control system 150 may control whenthe optical source 205 emits a pulse of light or a burst of light pulsesthat includes one or more pulses of light by sending one or more signalsto the optical source 205. The light beam 260 can include one or morebursts that are separated from each other in time. Each burst caninclude one or more pulses of light. In some implementations, a burstincludes hundreds of pulses, for example, 100-400 pulses.

The light beam 260 (and the light beam 160) are pulsed light beams. Thecoherence-reducing techniques discussed below may be applied on apulse-by-pulse basis. In other words, the amount of coherence (and,thus, the amount of speckle) may be controlled for each individualpulse. Additionally, the amount of speckle in each pulse may becontrolled based on a command signal from the lithography exposureapparatus 115. Prior to discussing the coherence reducing techniques,FIGS. 3A-3C provide an overview of the production of pulses in theoptical source 205. FIG. 3A shows an amplitude of a wafer exposuresignal 300 as a function of time, FIG. 3B shows an amplitude of a gatesignal 315 as a function of time, and FIG. 3C shows an amplitude of atrigger signal as a function of time.

The control system 150 can be configured to send the wafer exposuresignal 300 to the optical source 205 to control the optical source 205to produce the light beam 260. In the example shown in FIG. 3A, thewafer exposure signal 300 has a high value 305 (for example, 1) for aperiod of time 307 during which the optical source 205 produces burstsof pulses of light. The wafer exposure signal 300 otherwise has a lowvalue 310 (for example, 0) when the wafer 120 is not being exposed.

Referring to FIG. 3B, the light beam 260 is a pulsed light beam, and thelight beam 260 includes bursts of pulses. The control system 150 alsocontrols the duration and frequency of the bursts of pulses by sending agate signal 315 to the optical source 205. The gate signal 315 has ahigh value 320 (for example, 1) during a burst of pulses and a low value325 (for example, 0) during the time between successive bursts. In theexample shown, the duration of time at which the gate signal 315 has thehigh value is also the duration of a burst 316. The bursts are separatedin time by an inter-burst time interval. During the inter-burst timeinterval, the lithography scanning apparatus 115 may position the nextdie on the wafer 120 for exposure.

Referring to FIG. 3C, the control system 150 also controls therepetition rate of the pulses within each burst with a trigger signal330. The trigger signal 330 includes triggers 340, one of which isprovided to the optical source 205 to cause the optical source 205 toproduce a pulse of light. The control system 150 can send a trigger 340to the source 205 each time a pulse is to be produced. Thus, therepetition rate of the pulses produced by the optical source 205 (thetime between two successive pulses) can be set by the trigger signal330.

As discussed above, when the gain medium 219 is pumped by applyingvoltage to the electrodes 217, the gain medium 219 emits light. Whenvoltage is applied to the electrodes 217 in pulses, the light emittedfrom the medium 219 is also pulsed. Thus, the repetition rate of thepulsed light beam 260 is determined by the rate at which voltage isapplied to the electrodes 217, with each application of voltageproducing a pulse of light. The pulse of light propagates through thegain medium 219 and exits the chamber 214 through the output coupler218. Thus, a train of pulses is created by periodic, repeatedapplication of voltage to the electrodes 217. The trigger signal 330,for example, can be used to control the application of voltage to theelectrodes 217 and the repetition rate of the pulses, which can rangebetween about 500 and 6,000 Hz for most applications. In someimplementations, the repetition rate can be greater than 6,000 Hz, andcan be, for example, 12,000 Hz or greater

The signals from the control system 150 can also be used to control theelectrodes 217, 241 within the master oscillator 212 and the poweramplifier 230, respectively, for controlling the respective pulseenergies of the master oscillator 212 and the power amplifier 230, andthus, the energy of the light beam 260. There may be a delay between thesignal provided to the electrodes 217 and the signal provided to theelectrodes 241. The amount of delay may influence the amount ofcoherence in the pulsed light beam 260. For example, the coherence of apulse of the seed light beam 224 may vary in time, with the front edge(the portions of the pulse that occur first in time) having the leastcoherence and the later-occurring portions of the pulse having the mostcoherence. The delay between the signal provided to the electrodes 217and the signal provided to the electrodes 241 determines which portionof the pulse is amplified. Thus, a larger delay results in a pulse withmore coherence and a shorter delay results in a pulse with lesscoherence.

The pulsed light beam 260 can have an average output power in the rangeof tens of watts, for example, from about 50 W to about 130 W. Theirradiance (that is, the average power per unit area) of the light beam260 at the output can range from 60 W/cm² to 80 W/cm².

Referring to FIG. 4, a block diagram of an example optical lithographysystem 400 is shown. The optical lithography system 400 includes anoptical system 405 that provides a light beam 460 to the lithographyexposure apparatus 115. The optical system 405 may be similar to thepower amplifier (PA) 230 discussed with respect to FIG. 2. The opticalsystem 405 receives an input light beam 424 at a beam coupling opticalsystem 432. The input light beam 424 may be produced by a laser orsource that is similar to the master oscillator (MO) 212 of FIG. 2. Insome implementations, a master oscillator similar to the MO 212 isincluded in the optical lithography system 400.

In the implementation shown in FIG. 4, the beam coupling optical system432 includes an optical coupler 433 and a reflective optical element434. The optical coupler 433 is made of a material that transmits atleast some of the wavelengths in the input light beam 424, and maytransmit all of the wavelengths in the input light beam 424. The inputlight beam 424 passes through the optical coupler 433 and is reflectedfrom the reflective optical element 434. The reflective optical element434 may be any type of optical element that is capable of reflecting thelight beam 424. For example, the reflective optical element 434 may be amirror. FIGS. 6-9 show examples of optical elements that may be used asthe optical element 434.

The optical coupler 433 and the reflective optical element 434 arearranged relative to each other and the direction of propagation of theinput light beam 424 such that the input light beam 424 passes throughthe optical coupler 433 and reflects from the optical element 434 into adischarge chamber 440, which includes a gain medium 419. The dischargechamber 440 and the gain medium 419 may be similar to the dischargechamber 240 and the gain medium 219, respectively, discussed above withrespect to FIG. 2.

In the example of FIG. 4, the optical coupler 433 and the reflectiveoptical element 434 are in physical contact with each other and angledsuch that the reflective optical element 434 directs the input beam 424into the discharge chamber 440. However, in other implementations, theoptical coupler 433 and the reflective optical element 434 may be angledrelative to each other without making physical contact.

After reflecting from the reflective optical element 434, the input beam424 propagates in the discharge chamber 440 and the gain medium 419 to abeam turning optical system 452. The beam turning optical system 452includes at least one optical element that is positioned to turn thebeam 424 such that the beam 424 changes propagation direction and passesthrough the gain medium 419 again. The beam 424 may pass through thegain medium 419 after being turned by the beam turning optical system452 along a path that does not coincide with the path of the beam 424 asit travels toward the beam turning optical system 452. In this way, thebeam 424 follows a closed ring path 443 (dashed line) through thedischarge chamber 440. For example, the beam turning optical system 452may include a mirror, a corner-cube reflector, a partially reflectiveoptical element, and/or a combination of such elements. FIGS. 10-13 showexamples of elements that may be used as the beam turning optical system452.

After being turned by the beam turning optical system 452, the inputbeam 424 propagates in the gain medium 419 toward the beam couplingoptical system 432 and is amplified. The input beam 424 impinges on theoptical coupler 433, which is at least partially transmissive to thewavelengths in the input beam 424, and a portion of the input beam 424passes through the optical coupler 433 as an output beam 460. The outputbeam 460 is provided to the lithography exposure apparatus 115. Aportion of the input beam 424 that is not transmitted through theoptical coupler 433 is reflected from the optical coupler 433 andtravels about the closed path 443 again.

The optical coupler 433, the reflective optical element 434, and thebeam turning optical system 452 define a resonator cavity 457. Thecavity 457 defines the closed path 443 on which the beam 424 propagates.In FIGS. 4 and 5A, the closed path 443 is represented with a dashedline. At least a portion of the closed path 443 passes through the gainmedium 419. The path 443 is closed because the light entering the path443 intersects and overlaps itself at one or more points along the path443. In the example of FIG. 4, the intersection occurs at the opticalcoupler 433. The closed path 443 may be a ring, a circular or ellipticalloop, a polygonal loop, or any other loop-like closed path that has atleast a portion that does not reflect back on itself. In someimplementations, such as the example of FIG. 4, a portion of the closedpath 443 may intersect another portion of the closed path 443 in thecavity 457.

The resonator cavity 457 also may include additional elements that arenot shown in FIG. 4. For example, the resonator cavity 457 may include adispersive optical system between the beam coupling optical system 432and the gain medium 419. The dispersive optical system receives the beam424 from the reflective element 434 and demagnifies the light, producinga spatially narrow beam that passes through the gain medium 419. Afterthe beam 424 is returned from the beam turning system 452 and passesthrough the gain medium 419, the dispersive optical system magnifies thebeam 424 prior to the beam 424 reaching the optical coupler 433. Thedispersive optical system may include a collection of dispersive opticalelements, such as prisms and/or gratings. The dispersive optical systemmay be part of the beam coupling optical system 432.

Referring to FIG. 5A, a block diagram of another exemplary opticallithography system 500, which includes an optical system 505, is shown.The optical lithography system 500 is the same as the opticallithography system 400 (FIG. 4), except the optical lithography system500 includes a control system 550. The control system 550 provides acommand signal, which may be either or both of the command signals 555and 557, to the optical system 505 to adjust the cavity 457 during useof the optical system 505.

The optical lithography system 500 may include an optical beammonitoring optical system 562 that provides a portion 551 of the beam460 to the control system 550. In implementations that include theoptical beam monitoring optical system 562, the control system 550 andthe optical beam monitoring optical system 562 implement a closed-loopfeedback system that is used to adjust the amount of coherence in thebeam 460 while the optical system 505 operates. Alternatively or inaddition to receiving the portion 551 from the optical beam monitoringoptical system 562, the control system may receive a command signal 553from the lithography exposure apparatus 115. The command signal 553 mayinclude data from the detector 122 that indicates an amount of coherencein the beam 460.

Regardless of whether the optical beam monitoring optical system 562 orthe control signal 553 provides data to the control system 550, thecontrol system 550 implements a closed-loop feedback system thatdetermines a value of a property of the beam 460 based on measured dataand adjusts the cavity 457 when the value indicates that the coherenceof the beam 460 exceeds an acceptable amount. The acceptable amount ofcoherence may be a range or a single value. The range may be defined bya threshold above which the coherence is unacceptable. The closed-loopfeedback system may be used to control the coherence of the beam 460 ona pulse-by-pulse basis.

The coherence (and divergence) of the beam 460 may be adjusted bycontrolling the modal content of the light beam 460. The light beam 460includes modes, which are discrete resonant conditions at leastpartially determined by the geometry of the cavity 457. The beam 460includes only those electromagnetic waves that match the modes that thecavity 457 is able to support. Thus, the modes that are supported by thecavity 457 collectively make up the modal content of the light beam 460.Referring also to FIGS. 5B and 5C, which are side cross-sectional viewsof the cavity 457 in the x-y and y-z planes, respectively, the resonatorcavity 457 supports longitudinal or axial modes (in the x-y plane andalong a longitudinal axis 413 of the cavity) and lateral or transversemodes (in the y-z plane, which is perpendicular to the longitudinal axis413).

The longitudinal modes may be considered to be a standing wave patternformed by electromagnetic waves in the cavity 457, with the frequenciesof the standing waves corresponding to the discrete frequencies includedin the beam 460. The standing waves that are supported by the cavity 457are at least partially determined by the cross-sectional geometry of thecavity 457 in the x-y plane. The lateral modes are at least partiallygoverned by the geometry of the cavity 457 in the y-z plane. The lateralmodes may have horizontal (y direction) and vertical components (zdirection), and the lateral modes determine the intensity distributionof the beam 460 in the y-z plane. Thus, by changing the geometry of thecavity 457 by, for example, by changing the shape of an optical elementthat interacts with light in the cavity 457, the modal content of thebeam 460 may be changed. For example, an element that is included in thereflective optical element 434 and/or the beam turning system 452 maychange shape by being adjusted as discussed with respect to FIGS. 6-13.

The modal content of the beam 460 also may be influenced by operatingconditions, such as the temperature in the cavity 457, the pressure inthe cavity 457, the repetition rate of the beam 460, and the duty cycleof the beam 460. The gain medium 419 is excited by dischargingelectrodes in the cavity 457, and the repetition rate and duty cycle ofthe beam 460 are determined by the frequency and duration of thedischarge. When electrodes discharge, acoustic waves form in the medium419. The acoustic waves cause pressure variations that perturb the indexof refraction of the medium 419. The perturbations of the index ofrefraction are local, thus, the acoustic waves may cause the index ofrefraction of the medium 419 to spatially vary in the discharge chamber440. The variations in the index of refraction may alter the directionof the beam 424 that propagates in the chamber 440, and the medium 419may be considered to act as an aperture in the cavity 457 with anunknown and/or varying transfer function. Because the medium 419 acts asan aperture in the cavity 457, the medium 419 also partially determinesthe modes that are able to propagate in the cavity 457. Theperturbations of the index of refraction of the medium 419 thus alsoaffect the modal content of the beam emitted from the cavity 457,thereby affecting the coherence and the divergence of the beam 460. Thecharacteristics of the perturbations, such as, for example, theamplitude, location, and amount of the perturbations may vary with theconditions in the cavity 457. As such, changing the repetition rate ofthe beam 460 or any other operating condition may change the coherenceof the beam 460 in an unpredictable manner. Thus, it may be desirable tomonitor the beam 460 to determine whether the cavity 457 should beadjusted to reduce the coherence of the beam 460 while the system 500 isoperating and producing the beam 460.

The control system 550 allows the beam 460 to be monitored and thecoherence of the beam 460 controlled through adjustment of thereflective optical element 434 and/or the beam turning optical system452 during operation of the system 500.

The control system 550 may receive commands through an input/output(I/O) interface 567. The commands may originate from the lithographyexposure apparatus 115 and/or come directly from an operator or anautomated process. The control system 550 also may receive a commandsignal 553, which may include data related to the beam 460, from thedetector 122. In some implementations, the control system 550 receivesdata related to the beam 460 from the metrology module 568. The controlsystem 550 provides command signals 555, 557 to either or both of thebeam turning system 452 and the beam coupling optical system 432,respectively.

The control system 550 includes an electronic processor 564, anelectronic storage 566, the input/output (I/O) interface 567, and themetrology module 568. The metrology module 568 receives the portion 551of the beam 460 from the optical beam monitoring optical system 562 anddetermines a property of the beam 460 based on the portion 551. Themetrology module 568 may include any optical element, a collection ofoptical elements, and/or instrumentation that measures one or moreproperties of the beam 460. For example, the metrology module 568 mayinclude a camera that images the portion of the beam 460 provided by theoptical beam monitoring optical system 562. Images from the camera maybe used to determine properties of the beam 460 such as beam divergenceand intensity. The metrology module 568 may include other elements thatare able to assess the properties of the beam 460. For example, themetrology module 568 may include variable apertures, pinholes, and otherspatial filters that may be used to measure beam divergence. Themetrology module 568 also may include other optical elements such asmirrors and lenses that receive and direct the light from the opticalbeam monitoring optical system 562.

The optical beam monitoring optical system 562 may include any devicecapable of directing a portion of the beam 460 to the metrology module568. For example, the optical beam monitoring optical system 562 may bea beam splitter that directs some of the beam 460 toward the control. Inthe example shown in FIG. 5A, the beam monitoring optical system 562obtains a sample of the beam 460 that has passed through the beamcoupling optical system 432. However, the beam monitoring optical system562 may be placed in other locations. For example, the beam monitoringoptical system 562 may be inside the lithography exposure apparatus 115.In some implementations, the optical lithography system 500 includes abandwidth analysis module (BAM) between the beam coupling optical system432 and the lithography exposure apparatus 115. The BAM may be similarto the bandwidth analysis module 262 of FIG. 2, and the BAM measuresvarious parameters of the beam 560, such as, for example, the bandwidthand/or the wavelength. In these implementations, the beam monitoringoptical system 562 may be placed between the BAM and the lithographyexposure apparatus 115.

The control system 550 also includes the electronic processor 564, theelectronic storage 566, and the I/O interface 567. The electronicprocessor 564 includes one or more processors suitable for the executionof a computer program such as a general or special purposemicroprocessor, and any one or more processors of any kind of digitalcomputer. Generally, an electronic processor receives instructions anddata from a read-only memory, a random access memory, or both. Theelectronic processor 564 may be any type of electronic processor.

The electronic storage 566 may be volatile memory, such as RAM, ornon-volatile memory. In some implementations, and the electronic storage566 includes non-volatile and volatile portions or components. Theelectronic storage 566 may store data and information that is used inthe operation of the control system 550 and/or components of the controlsystem 550. The information may be stored in, for example, a look-uptable or a database. For example, the electronic storage 566 may storedata that indicates values of various properties of the beam 460 underdifferent operating conditions and performance scenarios. For example,the electronic storage 566 may store beam divergence values thatcorrespond to optimal performance of the optical system 505 at differentrepetition rates and cavity 457 temperatures. Additionally oralternatively, the electronic storage 566 may store settings thatpertain to either or both of the beam turning system 452 and the beamcoupling optical system 432.

The electronic storage 566 also may store instructions, perhaps as acomputer program, that, when executed, cause the processor 564 tocommunicate with components in the control system 550, the opticalsystem 505, and/or the lithography exposure apparatus 115. For example,the instructions may be instructions that cause the electronic processor564 to determine an amount of divergence of the beam 460 based oninformation from the metrology module 568. In some implementations, theinstructions are instructions that cause the control system 550 togenerate and provide a command signal to one or more components of thebeam turning system 452 or the beam coupling optical system 432 to movethe component to modify the resonator cavity 457 and reduce thecoherence of the beam 460. Moving the component may include any type ofmotion of the component or a portion of the component (such as a surfaceof the component that interacts with light). For example, moving mayinclude one or more of shifting, rotating, turning, lateral motion ofall or a portion of the component, any kind of shape change including adeformation or a change in an amount of curvature, and a change in anangle relative to incident light.

The I/O interface 567 is any kind of electronic interface that allowsthe control system 550 to receive and/or provide data and signals withan operator, the optical system 505, the beam turning system 452, thebeam coupling optical system 432, the lithography exposure apparatus115, and/or an automated process running on another electronic device.For example, the I/O interface 567 may include one or more of a visualdisplay, a keyboard, and a communications interface.

FIGS. 6-13 show various implementations of the reflective opticalelement 434 and the beam turning optical system 452. Any of thereflective optical elements shown in FIGS. 6-9 may be used as thereflective optical element 434, and any of the beam turning opticalsystems shown in FIGS. 10-13 may be used as the beam turning opticalsystem 452. An optical system may include any of the reflective opticalelements shown in FIGS. 6-9 as the reflective optical element 434 and/orany of the beam turning optical systems shown in FIGS. 10-13 as the beamturning optical system 452. In other words, any of the reflectiveoptical elements shown in FIGS. 6-9 may be used with any of the beamturning optical systems shown in FIGS. 10-13.

FIGS. 6-9 are side views of reflective optical elements 634, 734, 834,934, respectively. Each of the reflective optical elements 634, 734,834, 934 has a reflective surface that is oriented to receive the lightbeam 424, and the surface has a surface profile that determines how thesurface reflects incident light. The surface profile may be determinedby the radius of curvature of the surface, the material of the surface,and/or by physical characteristics of the surface (for example, thesurface may include a blazed grating). Because the reflective opticalelement 434 forms part of the resonator cavity 457, the surface profileof the optical element 434 determines the modal content of the light inthe resonator cavity 457.

FIG. 6 is a side view of the optical element 634 in the x-y plane. Theoptical element 634 extends in the z direction (into and/or out of thepage) along an axis 679. The optical element 634 includes a body 672,which has an optically reflective surface 674. The optically reflectivesurface 674 may be any type of material that reflects the wavelengths inthe beam 424. For example, the optically reflective surface 674 may be amirror or a multi-layer dielectric stack designed to reflect thewavelengths in the beam 424.

The surface 674 defines a surface normal 671. The reflective surface 674reflects incident light (including the beam 424) along the surfacenormal 671. The reflective surface 674 is curved in the x-y plane, thus,the surface normal 671 extends in more than one direction in the x-yplane. The reflective surface 674 may be, for example, a toroidalreflective surface (such as a cylindrical reflective surface) or anothertype of aspheric reflective surface. In the example shown in FIG. 6, thereflective surface 674 has a curvature that results in the surface 674being convex relative to the beam 424 in the x-y plane, and the surfacenormal 671 extends radially outward from the surface 674.

The optical element 634 also includes one or more rigid fasteners 675.The body 672 is capable of being deformed in response to an appliedforce 676. The applied force 676 may be, for example, a mechanical forceprovided by a piezoelectric (PZT) actuator and/or a pneumatic pressureapplied to a pneumatic cylinder. The applied force 676 may be any kindof a force, including a torque. The rigid fasteners 675 hold a portionof the body 672 at a fixed location when the force 676 is applied. Byholding a portion of the body 672 at a fixed location while the force676 is applied, the rigid fasteners 675 allow another portion of thebody 672 not held by the rigid fasteners 675 to deform (for example,change shape) in response to the application of the force 676.

The rigid fasteners 675 may be rigid posts that hold a portion of thebody 672 in a fixed location by making physical contact with theportion. In the example of FIG. 6, the rigid fasteners 675 include twoposts. One of the posts is attached to the body 672 at an end 677 a, andthe other post is attached to the body 672 at an end 677 b.

In the example shown in FIG. 6, the force 676 is applied to a side 678of the body 672. The side 678 is opposite to the surface 674. The force676 is applied along a direction that causes the curvature of thesurface 674 to change. In the example shown, the force 676 is applied atthe center of the body 672 and in a direction that is parallel to thesurface normal 671 that extends from the center of the surface 674. Byapplying the force 676 at the side 678, the surface 674 is deformed. Forexample, the radius of curvature of the surface 674 may change by 1-10microns (μm) in response to the application of the force 676. The amountof deformation is determined by the amount of force applied at the side678. Thus, the curvature of the surface 674 may be varied by applying anappropriate about of force at the side 678.

The optical element 634 may be used as the reflective optical element434 in the optical lithography system 400 or the optical lithographysystem 500. In the optical system 400, the optical element 434 does notreceive command signals from a control system. When used in the opticallithography system 400, the reflective surface 674 of the opticalelement 634 is deformed to have a particular curvature, for example,when the optical lithography system 400 is assembled, or duringmaintenance, and the deformation is not purposefully adjusted duringoperation of the system 400.

When used in the optical lithography system 500, the optical element 434may receive the command signal 557 from the control system 550. Theoptical element 434 responds to an instance of the command signal 557 bymoving or adjusting. For example, the command signal 557 may includeinformation that determines how much force is to be applied to the body672, and the command signal 557 may be used to apply a particular amountof force at the side 678. The control system 550 provides the commandsignal 557 while the system 505 is operating, and may provide aninstance of the command signal 557 each time a pulse of light isproduced by the system 505. Thus, when the optical element 634 is usedin the optical system 505, the surface 674 may be deformed, and may bedeformed in a different way, for each pulse of light in the output beam460.

FIG. 7 shows a side view of a reflective optical element 734 in the x-yplane. The reflective optical element 734 extends in the z direction(into and/or out of the page). The reflective optical element 734includes a body 772. The body 772 includes a plurality of segments 781,each of which has a reflective surface 774. Each reflective surface 774may have a different surface profile. For example, one or more of thesegments 781 may have a reflective surface with a different curvaturethan the reflective surfaces associated with the other segments.Although the body 772 includes the segments 781, and each segment 781includes a reflective surface 774, the body 772 and surfaces 774 arephysically joined or touching such that the reflective surface 774 doesnot intentionally include any discontinuities or regions that do notreflect light between two adjoining segments 781.

In the example shown, the body 772 includes three segments 781 a, 781 b,781 c. The segments 781 a, 781 b, 781 c have corresponding reflectivesurfaces 774 a, 774 b, 774 c. At least one of the reflective surfaces774 a, 774 b, 774 c has a different amount of curvature than another ofthe segments. In some implementations, each of the reflective surfaces774 a, 774 b, 774 c have a different curvature.

FIG. 8 is a side view of a reflective optical element 834 in the x-yplane. The optically reflective element 834 includes a body 872. Thebody 872 includes a plurality of segments, each of which has areflective surface 874. The segments are joined such that the reflectivesurfaces form a continuous reflective surface that lacks regions that donot reflect light. In the example of FIG. 8, the body 872 includessegments 881 a, 881 b, 881 c. The segments 881 a, 881 c includerespective base portions 882 a, 882 c and reflective surfaces 874 a, 874c. The reflective surfaces 874 a and 874 c are gratings. The gratings874 a and 874 c may be identical or different. The segment 881 b is amirror that has a reflective surface 874 b. The mirror may be acylindrical mirror 881 b that extends in the z direction (into and/orout of the page). The reflective surface 874 b is curved. In the exampleof FIG. 8, the reflective surface 874 b is convex in the x-y planerelative to the beam 424. The reflective surface 874 (which is thecollection of reflective surfaces 874 a, 874 b, 874 c) is also convex inthe x-y plane relative to the beam 424.

The reflective optical element 834 may be used in the system 400 as thereflective optical element 434.

FIG. 9 is a side view of a reflective optical element 934, whichincludes a reflective surface 974, in the x-y plane. The reflectiveoptical element 934 is similar to the reflective optical element 834(FIG. 8), except that the surface profile of the reflective surface 974may be changed while the reflective optical element 934 is being used.

The reflective optical element 934 includes a body 972 that has areflective surface 974. The body 972 includes segments 981 a, 981 b, 981c, which have respective bases 982 a, 982 b, 982 c and reflectivesurfaces 974 a, 974 b, 974 c. The segments 981 a, 981 b, 981 c arejoined such that the reflective surface 974 (which is made up of thereflective surfaces 974 a, 974 b, 974 c) is a continuous reflectivesurface that does not include any regions that are not opticallyreflective.

The reflective optical element 934 includes flexible connectors 983. Oneof the connectors 983 is between the segments 981 a and 981 b, andanother flexible connector 983 is between the segments 981 b and 981 c.One of the flexible connectors 983 connects the segments 981 a and 981b, and another of the flexible connectors 983 connects the segments 981b and 981 c. The flexible connectors 983 connect the various segments ona side 978 such that the flexible connectors 983 do not impact thesurface profile of the reflective surface 974. The flexible connectors983 hold the segments 981 a-981 c allow the segments 981 a-981 c to moveindependently of each other in response to a force applied to aparticular segment. However, the flexible connectors 983 have sufficientstrength and rigidity such that the segments 981 a-981 c do not moverelative to each other when a force is not applied. The flexibleconnectors 983 may be, for example, grooves that are formed between theportions. In the x-y plane, the grooves open to the side 978. Thegrooves may be v-shaped grooves that open to the side 978 and taper to apoint in the x-y plane toward the reflective surface 974 (such as shownin FIG. 9). In some implementations, the grooves may be rounded groovesthat are open to the side 978 and are rounded toward the reflectivesurface 974. Rounded grooves may distribute mechanical stress moreeffectively than other types of flexible connectors, and may result inthe reflective optical element 934 being less prone to fracture whenforce is applied to one of more of the segments.

In the example of FIG. 9, the segment 981 b is fixed in place by a post975 attached to the segment 981 b at the side 978. Independent forces976 a, 976 c may be applied to segments 981 a, 981 c, respectively. Theforces 976 a, 976 c may be applied using, for example, a PZT transducer.The forces 976 a, 976 c are applied in the x-y plane from the side 978.In the example shown in FIG. 9, the forces 976 a, 976 b are in adirection that is toward the respective surfaces 974 a, 974 c. However,the forces 976 a, 976 b may be applied in other directions in the x-yplane.

Application of the force 976 a to the segment 981 a causes the segment981 a and the reflective surface 974 a to move relative to the segment981 b. Application of the force 976 c to the segment 981 c causes thesegment 981 c and the reflective surface 974 c to move relative to thesegment 981 c. By moving either or both of the reflective surfaces 974a, 974 c, the light that is reflected from the surfaces 974 a, 974 c maybe steered into the gain medium 419 and toward the beam turning system252.

The reflective optical element 934 may be used in the system 500 as thereflective optical element 434 and the surface profile of the reflectivesurface 1374 of the reflective optical element 934 may be changed whilethe system 500 is in operation. For example, the surface profile of thereflective surface 974 may be changed for each pulse of the light beam424.

FIGS. 10-13 are side views of beam turning systems 1052, 1152, 1252,1352 all of which are examples of implementations of the beam turningsystem 452. The beam 424 that propagates in the resonator cavity 457 maybe represented by optical rays. In the examples of FIGS. 10-13, the raylabeled 424 a is a ray that represents the propagation of the beam 424when the modal content of the beam has not been changed to reduce thecoherence of the beam 460. For example, the ray 424 a may arise when thereflective optical element 434 is a cylindrical mirror that is notadjustable, and the beam turning optical system 452 is a corner cubereflector that is not adjustable and not large enough in the x-y planeto accommodate the ray 424 b.

The ray 424 b is a ray that propagates through the resonator cavity 457in a manner that increases the lateral modes as compared to the ray 424a. The ray 424 b may arise from using one of the reflective opticalelements of FIGS. 6-9 as the reflective optical element 434. FIG. 10shows an example beam turning system 1052 that is not adjustable but hasa sufficiently expansive reflective surface to accommodate the ray 424b. As discussed below, the beam turning systems of FIGS. 11-13 areadjustable to accommodate rays such as the ray 424 b. In this way, usingone of the beam turning systems of FIGS. 11-13 as the beam turningsystem 452 may allow the modal content of the beam 424 to be increased,thereby decreasing the coherence of the beam 424 and reducing thepresence of speckle.

Referring to FIG. 10, a side view of the beam turning system 1052 isshown. FIG. 10 shows the beam turning system 1052 in the x-y plane fromthe top side. The beam turning system 1052 includes a body 1092 and areflective surface 1094. The body 1092 includes two portions 1091 a,1091 b, which extend from a vertex 1093. The portions 1091 a, 1091 b areangled relative to each other. In the example of FIG. 10, the portions1091 a, 1091 b form a right angle at the vertex 1093. However, theportions 1091 a, 1091 b may form other angles at the vertex 1093. Thebody 1092 may be a single integral component, or the portions 1091 a,1091 b may be made from two separate portions that are physicallyjoined.

The two portions 1091 a, 1091 b include respective reflective surfaces1094 a, 1094 b. The ray 424 b propagates in the medium 419 and isincident on the reflective surface 1094 b, which reflects the ray 424 btoward the reflective surface 1094 a. The ray 424 b is reflected by thesurface 1094 a and directed back in to the medium 419. The ray 424 bpropagates in the medium toward the optical coupler 433 (FIGS. 4 and 5).

Referring to FIG. 11A, a side view of the beam turning system 1152 isshown. The beam turning system 1152 is similar to the beam turningsystem 1052 (FIG. 10), except the beam turning system 1152 includes asecondary portion 1191 c, which may be positioned or moved relative toportions 1191 a, 1192 b. In the example shown, the portions 1191 a, 1191b extend from a vertex 1193 at a right angle. The portion 1191 c isphysically connected to the portion 1191 b, but is movable relative tothe portion 1191 b. For example, the portions 1191 b and 1191 c may beconnected by a flexible portion 1183, which may be similar to theflexible portion 983 discussed above with respect to FIG. 9.

Each of the portions 1191 a, 1191 b, 1191 c includes a respectivereflective surface 1194 a, 1194 b, 1194 c. The reflective surfaces 1194a, 1194 b, 1194 c collectively form a reflective surface 1194 of thebeam turning system 1152. The reflective surface 1194 is a continuousreflective surface. Light from the medium 419 is incident on thereflective surface 1194 b or 1194 c, and these surfaces reflect theincident light to the reflective surface 1194 a. The reflective surface1194 a directs the light back into the medium 419 and toward the opticalcoupler 433.

Referring also to FIG. 11B, the secondary portion 1191 c may bepositioned relative to the portion 1191 b by moving the secondaryportion 1191 c in the x-y plane along an arc A while the portions 1191b, 1191 c are connected by the flexible portion 1183. In this way, theangle between the reflective surfaces 1194 b, 1194 c may be changed. Theangle is shown as angle B in FIG. 11. By changing the angle B, lateralmode growth in the resonator cavity 457 may be enhanced. For example,decreasing the angle B may result in the beam turning system 1152 beingable to reflect more rays (such as the ray 424 b) that arise fromadditional lateral modes.

The beam turning system 1152 may be used as the beam turning system 452in the system 400 or the system 500. In some implementations, such as,for example, when the beam turning system 1152 is used in the system400, secondary portion 1191 c is positioned relative to the portion 1191b when the resonator cavity 457 is assembled or during maintenance andthe angle B is not intentionally changed during operation of the system400.

In some implementations, the secondary portion 1191 c may be movedrelative to the portion 1191 b during operation of an optical systemthat includes the beam turning system 1152. For example, the beamturning system 1152 may be used as the beam turning system 452 in thesystem 500. In these implementations, the beam turning system 1152includes an actuator that causes the portion 1191 to move in response toreceiving the command signal 555. The actuator may be, for example, aPZT actuator that applies a force to the portion 1191 c to cause theportion 1191 to move along the arc A during operation of the opticalsystem. In these implementations, the portion 1191 c may be moved to adifferent position along the arc A, for example, each time a pulse oflight is produced. A correction to a pulse of light is based oninformation from a prior pulse. The prior pulse may be any pulse thatprecedes the pulse of light in time. For example, the prior pulse may bethe pulse that immediately precedes the pulse of light in time, and/or apulse that occurred before the immediately preceding pulse.

Referring to FIG. 12, a side view of a beam turning system 1252 isshown. The beam turning system includes a base body 1292 a, which ispositioned relative to a first wing body 1292 b and a second wing body1292 c. The first and second wing bodies 1292 b, 1292 c are notphysically connected to the base body 1292 a. The first wing body 1292 bis displaced from the base body 1292 a in the x and y directions. Thesecond wing body 1292 c is displaced from the base body 1292 a in the xand −y directions. The beam turning system 1252 may have mirror symmetryabout an axis 1213, which is parallel to the x direction. The base body1292 a, the first wing body 1292 b, and the second wing body 1292 cinclude reflective surfaces 1294 a, 1294 b, 1294 c, respectively.Together, the reflective surfaces 1294 a, 1294 b, 1294 c form areflective surface 1294. The reflective surface 1294 has gaps 1295 wherethe portions are separated. Thus, unlike the reflective surface 1194,for example, the reflective surface 1294 is not continuous.

The reflective surface 1294 c of the first wing body 1292 c receives theray 424 b from the gain medium 419 and directs the ray 424 b toward thereflective surface 1294 b of the second wing body 1292 b. The ray 424 bis reflected from the reflective surface 1294 b, back into the gainmedium 419 and toward the optical coupler 433. The reflective surfaces1294 b, 1294 c are positioned to receive and reflect, respectively, theray 424 b. In this way, the first wing body 1292 b and the second wingbody 1292 c enable additional lateral modes to propagate in theresonator cavity 457 as compared to a design that does not include thefirst and second wing bodies 1292 b, 1292 c. The beam turning system1252 may be used as the beam turning system 452 in the system 400.

Referring to FIG. 13, a side view of a beam turning system 1352 isshown. The beam turning system 1352 is the same as the beam turningsystem 1252 (FIG. 12), except force 1376 b, 1376 c, respectively, may beapplied to the first wing body 1292 b and the second wing body 1292 cwhile the beam turning system 1352 is in use and interacting with thebeam 424 (which includes the rays 424 a and 424 b). The force 1376 b andthe force 1376 c may be applied with, for example, a PZT actuator.

The beam turning system 1352 may be used in the system 500 as the beamturning system 452. The control signal 555 (FIG. 5) may be used tocontrol the position of the first wing body 1392 b and/or the secondwing body 1392 c.

Referring to FIG. 14, a flow chart of an exemplary process 1400 isshown. The process 1400 may be used to control an optical system. Forexample, the process 1400 may be used to control an optical system thatis part of an optical source (such as the source 105 of FIG. 1). Theprocess 1400 may be used with any optical system, such as, for example,the optical system 405 or 505. The process 1400 may be used with anoptical source (such as, for example, the optical source 105 or 205)that includes one or more optical systems. For example, the process 1400may be used to control the optical source 205 and/or the PA 230 of theoptical source 205. The process 1400 may be used implemented in aclosed-loop control system, such as the system 500 (FIG. 5). The process1400 is discussed first with respect to the system 500 of FIG. 5, whichincludes the optical system 505. The process 1400 may be performed byone or more electronic processors 564 of the control system 550 whilethe system 505 produces the light beam 460. The process 1400 may beperformed on a pulse-by-pulse basis such that the process 1400 isperformed on a particular pulse and the immediately subsequent pulse, oron all pulses within a subset of pulses. A subset of pulses is any setof pulses in the beam 460, and may be composed of non-consecutivepulses.

A measured value of a property of a particular pulse of the light beam460 is accessed (1410). The particular pulse may be any pulse in thelight beam 460. The property is related to an amount of coherence in theparticular pulse. For example, the property may be a divergence of theparticular pulse along the direction of propagation or an intensityprofile of the pulse in a plane that is perpendicular to the directionof propagation of the pulse. The measured value of the property may beobtained from the metrology module 568, which receives a portion 551 ofthe light beam 460 from the optical beam monitoring optical system 562,or from the detector 122, which is in the lithography exposureapparatus. In some implementations, the measured value of the propertyof the particular pulse may be accessed from the electronic storage 566.

The measured value of the particular pulse may be measured at any pointalong the propagation path of the particular pulse. For example, themeasured value may be the value of the property outside of the cavity457 (for example, at the optical beam monitoring optical system 562), orthe measured value may be a value of the property inside of the cavity457. In some implementations, the measured value is obtained or derivedfrom data collected by the detector 122, which is in the lithographyexposure apparatus 115. Using data from the detector 122 may provideadditional flexibility and accuracy when exposing microelectronicfeatures that are on the edges of the wafer 120 (FIG. 1). Additionally,using data from the detector 122 may eliminate the need for the opticalbeam monitoring optical system 562.

The accessed measured value of the property is compared to a targetvalue of the property (1420). The target value of the property may be avalue of the property associated with optimal, proper, and/or acceptableoperation of the optical system 505. The target value may be stored onthe electronic storage 566. The measured value and the target value maybe compared by, for example, subtracting the measured value and thetarget value to determine an error value for the particular pulse.

A control signal may be generated based on the comparison (1430). Acontrol signal may be generated when the error value or other metricthat represents the magnitude difference between the target value andthe measured value exceeds a threshold. The threshold may encompass arange of values for the property about the target value. Thus, dependingon the threshold, measured values that are close to but less than thetarget value or greater than but not the same as the target value maynot result in the generation of a control signal. If the measured valueand the target value are the same, then a control signal is notgenerated. The measured values may be determined from measuring thevalue of a property (such as divergence or intensity) for each pulse andaveraging these values. For example, the value of the property may bemeasured for each pulse in a burst of pulses and the values averagedover the entire burst. In some implementations, the value may bemeasured for a plurality of pulses (for example, a group of pulses) thatis other than all of the pulses in a burst. For example, the value maybe measured for every other pulse in a burst, or for a subset ofconsecutive pulses in a burst (for example, a window of pulses). Inanother example, the value may be measured for all of the pulses in afirst burst of pulses and for fewer than all in the next burst ofpulses.

If a control signal is generated, the amount of coherence in the lightbeam 460 is adjusted by modifying the optical system 505 based on thecontrol signal to reduce the amount of coherence in a pulse that issubsequent to the particular pulse (1440). The generated control signalmay be the control signal 557, which is provided to the reflectiveoptical element 434, and/or the control signal 555, which is provided tothe beam turning optical system 452. The optical system 505 may bemodified by moving or positioning all or a portion of either or both ofthe reflective optical element 434 and the beam turning optical system452 based on the generated control signal.

For example, the reflective optical element 634 (FIG. 6) may be used asthe element 434, and the beam turning system 1352 (FIG. 13) may be usedas the beam turning system 452. In this example, a property of the beam460 determined by the metrology module based on the portion 551 receivedfrom the beam monitoring optical system 562. It is determined thatmagnitude of the difference between the value of the property of thebeam 460 and the target value of the property exceeds the threshold. Thecommand signal is generated, and, in this example, the command signalincludes the command signal 555, which is provided to the beam turningsystem 1352, and the command signal 557, which is provided to thereflective optical element 634.

The command signal 557 includes information sufficient to cause theforce 676 (FIG. 6) to be applied to the side 678 of the reflectiveoptical element 634. The amount of force 676 to be applied may be basedon the amount of the difference between the measured value of theproperty and the target value. When the force 676 is applied to thereflective optical element 634, the surface 674 moves. Because portionsof the surface 674 are held by the fasteners 675, moving the surface 674changes the curvature of the surface 674. By adjusting the curvature ofthe surface 674, the cavity 457 is modified such that additional lateralmodes are able to propagate in the cavity 457. This results in the beam460 having a greater divergence and less coherence.

Similarly, the command signal 555, which is provided to the beam turningsystem 1352, includes information sufficient to cause either or both ofthe forces 1376 b, 1376 c (FIG. 13) to be applied to the first andsecond wing bodies 1392 b, 1392 c, respectively. The applied forceresults in the first and/or second wing bodies 1392 b, 1392 c movingrelative to the base body 1392 a. By moving the first wing body 1392 band/or the second wing body 1392 c, the cavity 457 may be furthermodified to accommodate additional lateral modes, and the coherence ofthe beam 460 is decreased.

Additionally, when a control signal is generated, the value of theproperty may be measured again after the optical system 505 is modifiedbased on the control signal to obtain a new value of the property. Thenew value of the property is compared to the target value. If themagnitude of the difference between the new value of the property andthe target value still exceeds the threshold, a second command signal isgenerated and the optical system 505 is further modified. The process ofmeasuring the value of the property after applying a generated controlsignal to the optical system 505 may be repeated. In someimplementations, the process of measuring the property after applying agenerated control signal may be repeated until the magnitude of thedifference between the target value and the measured value is below thethreshold, indicating that the coherence of the beam 460 is less than anacceptable amount. In some implementations, the value of the propertymay be measured, and a control signal may be generated and provided tothe optical system 505 for each pulse in the beam 460.

If a control signal is not generated, then the measured value of theproperty is close to or equal to that of the target value. When themeasured value of the property is close to or equal to that of thetarget value, the optical system 505 may be operating as expected,optimally, or acceptably and the beam 460 may have a relatively lowamount of noise in the form of speckle. Thus, when a control signal isnot generated, the value of the property may be a value that can be usedas a target value at the particular operating conditions of the opticalsystem 505. The operating conditions may include, for example,temperature in the cavity 457, pressure in the cavity 457, repetitionrate, duty cycle, lifetime accumulated pulse count for the cavity 457,accumulated pulse count for the cavity 457 since the most recent gasinjection, and total hours of operation. The value of the property whenthe command signal is not generated may be stored in the electronicstorage in associated with one or more parameters that indicate theoperating conditions. The operating parameters may include one or moreof the temperature, the repetition rate, the duty cycle, a positioningof the reflective optical element 434 and/or the beam turning opticalsystem 452, and an amount of force applied to the reflective opticalelement 434 and/or the beam turning optical system 452. The stored valuemay be used as the target value in future instances of operating theoptical system 505 under the same conditions.

In this way, in addition being able to adjust the coherence of the beam460, the process 1400 also may improve the performance of the opticalsystem 505 by allowing the optical system 505 to quickly be adjusted toperform optimally under a wide range of operating conditions. Forexample, by storing parameters related to the measured value of theproperty at different repetition rates, the optical system 505 may relyon these stored values to quickly adapt to producing low coherencepulses after receiving a command signal 553 from the lithographyexposure apparatus 115 or a command through the I/O interface 567 froman operator to change the repetition rate of the beam 460.

Referring to FIG. 15, a flow chart of an example of a process 1500 forcontrolling an optical system is shown. The process 1500 may be used toimplement closed-loop control of an optical system to monitor thecoherence of an optical beam produced by the optical system (or anoptical beam produced by an optical source that includes the opticalsystem) to ensure that the coherence does not add excessive noise in theform of speckle to the optical beam. The process 1500 modifies an aspectof the optical system to reduce the coherence when the amount ofcoherence exceeds a predetermined amount. For example, the process 1500may modify a physical property of an optical element in the opticalsystem.

The process 1500 may be used with any optical source, such as, forexample, the optical source 105 or 205. The process 1400 may be usedwith an optical source (such as, for example, the optical source 105 or205) that includes one or more optical systems. For example, the process1400 may be used to control the optical source 205 and/or the PA 230 ofthe optical source 205. The process 1400 may be used with the opticalsystem 405 or 505. The process 1500 is discussed with respect to thesystem 500 of FIG. 5. The process 1500 may be performed by one or moreelectronic processors 564 of the control system 550 while the system 500produces the light beam 460. The process 1500 may be performed on apulse-by-pulse basis such that the process 1500 is performed on aparticular pulse and the immediately subsequent pulse, or on all pulseswithin a subset of pulses, where the subset of pulses is any set ofpulses in the beam 460.

A value of a property of the optical beam 460 is obtained (1510). Theoptical beam 460 is a pulsed optical beam produced by the optical system505. The property may be any property that is related to an amount ofcoherence in the light beam 460, or any property from which the amountof coherence of the light beam 460 may be derived or determined. Forexample, the property may be a divergence of the light beam 460 or avalue that characterizes the intensity profile of the light beam 460 ina plane perpendicular to the direction of propagation of the light beam460. In some implementations, the obtained value is obtained when theoptical system 505 is performing at least as expected and perhaps isperforming in an ideal or optimal fashion. Thus, in theseimplementations, the obtained values may be values that correspond to anacceptable amount of coherence being present in the beam 460.

The value of the property is obtained at more than one operatingcondition of the optical system 505. An operating condition of thesystem 505 is defined by one or more parameters, such as temperatureand/or pressure in the cavity 457, the repetition rate of the beam 460,the duty cycle of the beam 460, and status information related to anoptical element that includes an optical surface that interacts with thebeam 424 in the cavity. The optical element may be, for example, any ofthe reflective optical elements 634-934 (FIGS. 6-9) or the beam turningoptical systems 1052-1352 (FIGS. 10-13).

The status information related to the optical element may be anyinformation that indicates how the optical surface of the opticalelement interacts with the light that propagates in the cavity 457. Forexample, the status information may indicate a position of the opticalsurface relative to the beam 424, relative to other portions of theoptical element, and/or relative to other optical elements in thecavity. The status information may be an indication of an orientation ofthe optical surface relative to the light that propagates in the cavity457. For example, the status information may include an angle of theoptical surface relative to the longitudinal axis 413 (FIG. 5B) of thecavity or to a portion of the closed path 443. For optical elements thatinclude an optical surface that moves in response to application of aforce, such as the optical element 634 (FIG. 6), the optical element 934(FIG. 9), the beam turning optical system 1152 (FIG. 11), and the beamturning optical system 1352 (FIG. 13), the status information mayinclude an amount of force that is applied to the optical element.

The obtained value of the property and the status information are storedin the electronic storage 566 for each operating condition (1520). Theobtained value of the property and the amount of applied force arestored in association with parameters that define the operatingcondition. The obtained value of the property and the amount of appliedforce may be stored in a look-up table that is indexed by the operatingcondition parameters. In some implementations, the obtained value of theproperty and the amount of applied force are stored in a database thatis indexed by the operating condition parameters.

The optical system 505 is operated at a current operating condition(1530). The optical system 505 may be considered to be operating whenthe beam 460 is being produced. The current operating condition isdefined by the operating parameters of the optical system (such as therepetition rate of the beam 460 and the temperature of the gain medium419) and status information of the optical element while the beam 460 isbeing produced. The current operating condition may be a condition thatis the operating condition for any number of pulses in the beam 460. Forexample, the operating condition may apply to all of the pulses in aburst of pulses, for a plurality of bursts, or for a single pulse. Forexample, the repetition rate of the beam 460 may define the operatingcondition of the optical system 505.

An indication of a change in the current operating condition is received(1540). The indication of a change includes a new value of one or moreoperating parameters of the optical system 505. For example, theindication of a change may include a new repetition rate for the beam460. Thus, in this example, the new value is a new repetition rate. Theindication of a change may be received from the lithography exposureapparatus 115 via the command signal 553. Thus, the lithography exposureapparatus 115 may command that the optical system 505 change one or moreof the parameters such that the optical system 505 has a new operatingcondition. The indication of the change in the current operatingcondition may be received from an operator of the system 500. Forexample, the operator may enter the parameters associated with the newoperating condition directly into the I/O interface 567.

Status information related to the optical element and associated withthe new value of one or more of the operating parameters is determined(1550). The status information associated with the new value may bedetermined from the data stored in (1520). For example, the new valuemay be a new repetition rate, and the status information related to theoptical element that is associated with the new repetition rate may beretrieved from the stored data. If the stored data does not includestatus information for the new value, status information associated withvalues of the parameter that are most similar to the new value may beused to estimate the status information for the new value. For example,if status information related to the optical element is known forseveral repetition rates greater than and less than the new repetitionrate, the status information related to the optical element may beestimated from the status information that is available. For example,the status information may be tip-tilt information that describes theposition of the optical surface at several different repetition rates.The tilt is the amount of deviation in the direction of propagationprovided by the optical surface, and the tip is a direction that isorthogonal to the tilt. The tip-tilt information may be averaged or fitto a polynomial to estimate the tip-tilt values for the new repetitionrate.

A command signal based on the determined status information is appliedto the optical element (1560). The command signal may be, for example,the command signal 555, which is provided to the beam turning opticalsystem 452, and/or the command signal 557, which is provided to thereflective optical element 434. Applying the command signal to theoptical element may cause the optical surface of the optical element 434to move in accordance with the command signal. For example, the commandsignal may be sufficient to drive a PZT actuator that causes the opticalsurface to move relative to other elements in the cavity 457 or to havea different curvature.

In some implementations, another value of a property of the light beam460 is obtained after the command signal is provided to the opticalelement to determine whether the coherence of the beam 460 has beenreduced as a result of applying the command signal to the opticalelement. The obtained other value may be compared to a value of theproperty known to be associated with an acceptable amount of coherence.If the obtained other value is not indicative of an acceptable amount ofcoherence, additional command signals may be applied and the valuemeasured until the value of the property is indicative of an acceptableamount of coherence. For example, the additional command signals mayhave the effect of causing the optical element to be dithered about theinitial position until the optical surface is positioned such that thevalue of the property indicates an acceptable amount of coherence in thebeam 460.

When the value of the property indicates an acceptable amount ofcoherence in the beam 460, the status information related to the opticalelement may be stored in the electronic storage 566. For example, thestatus information may replace or update the status information that waspreviously stored in association with the operating condition in thelook-up table or database. In this way, the process 1500 allowsadaptation to changing circumstances that can affect the performance ofthe optical system 505 in a given operation condition. For example,components, such as the reflective optical element 434, the beam turningoptical system 452, and/or the medium 419 may degrade over time suchthat the previously determined status information for a given operatingcondition becomes inaccurate.

In the process 1500, (1510) and (1520) may be performed at a differenttime using the system 505 than (1530-1560). In other words, the storeddata that associates the status information of the optical element andthe operating conditions may be created a prior and used at a later timeto operate the system 505. The stored data may be used repeatedly.Additionally, as discussed above, the stored data may be updated toaccount for changing circumstances of the system 505.

Other implementations are within the scope of the claims.

What is claimed is:
 1. A system for a light source, the system comprising: an optical coupler; a first optical system comprising a first optical element; and a second optical system comprising a second optical element, wherein the first optical system and the second optical system define a space configured to hold a gain medium, the first optical element, the second optical element, and the optical coupler define a closed path, at least a portion of the closed path passing through space configured to hold the gain medium, and the first optical element and/or the second optical element comprise an optically reflective surface positioned to interact with a light beam that propagates on the closed path, the surface comprising a plurality of portions, at least one of the portions having a surface profile that is different from the surface profiles of the other segments.
 2. The system of claim 1, wherein the first optical element comprises the surface comprising a plurality of portions, at least one of the portions comprising a mirror and at least one of the portions comprising a grating.
 3. The system of claim 1, wherein the portions are adjacent to each other, and a flexible member is between any two adjacent portions such that at least one portion is movable relative to an adjacent portion.
 4. The system of claim 3, wherein any portion is movable relative to an adjacent portion.
 5. The system of claim 3, wherein the surface profile of a portion is determined by a radius of curvature and/or a surface characteristic of the portion.
 6. A system for a light source, the system comprising: an optical coupler; a first optical system comprising a first reflective optical element; and a second optical system comprising a second reflective optical element, wherein the first optical system and the second optical system define a space configured to hold a gain medium, the first reflective optical element, the second reflective optical element, and the optical coupler define a closed path, at least a portion of the closed path passing through the gain medium, the second reflective optical element comprises a plurality of distinct reflective surfaces, a first of the distinct reflective surfaces positioned to receive a light beam from the gain medium and to reflect the light beam to a second of the distinct reflective surfaces, the second of the distinct reflective surfaces being positioned to receive the light beam from the first distinct reflective surface and to reflect the light beam into the gain medium, and at least one of the distinct reflective surfaces being physically separated from the other reflective surfaces and/or movable relative to the other reflective surfaces.
 7. The system of claim 6, wherein at least one of the distinct reflective surfaces of the second optical element is movable relative to the other reflective surfaces, and the second optical system further comprises an actuator coupled to the at least one movable reflective surface, the actuator being configured to cause the movable reflective surface to move in response to a command signal.
 8. A system comprising: an optical system configured to amplify a pulsed optical beam, the pulsed optical beam having a repetition rate, the optical system comprising: an optical coupler; a first optical element; and a second optical element, the first optical element and the second optical element defining a space configured to hold a gain medium, and the first optical element, the second optical element, and the optical coupler defining a closed path that passes through the space configured to hold the gain medium; a lithography exposure apparatus configured to receive the amplified pulsed optical beam; and a control system coupled to the optical system and the lithography exposure apparatus, the control system configured to: receive a request to change the repetition rate of the pulsed optical beam from a first repetition rate to a second repetition rate, determine a value of a property of the pulsed optical beam at the second repetition rate, determine whether the value of the property is acceptable, and if the value of the property is not acceptable, generate a command signal based on the determined value of the property, and provide the command signal to the optical system, the command signal being sufficient to physically modify a controllable optical element that interacts with light that propagates on the closed path of the optical system.
 9. The system of claim 8, wherein the value of the property of the pulsed optical beam is a measure of coherence of one or more pulses of the pulsed optical beam.
 10. The system of claim 8, wherein the control system is further configured to determine whether the value of the property is acceptable after providing the command signal to the optical system.
 11. The system of claim 9, wherein determining a value of the property of the pulsed optical beam at the second repetition rate comprises accessing a value of an amount of force applied to the controllable optical element, and wherein providing the command signal to the controllable optical element comprises applying a different amount of force to the controllable optical element.
 12. The system of claim 9, wherein determining a value of the property of the pulsed optical beam at the second repetition rate comprises measuring a divergence of the pulsed optical beam when the optical system operates at the second repetition rate.
 13. The system of claim 9, wherein determining a value of the property of the pulsed optical beam at the second repetition rate comprises accessing a value of the property of the pulsed optical beam stored in an electronic storage.
 14. The system of claim 13, wherein the value of the property of the pulsed optical beam stored in an electronic storage comprises a value measured previously and while the optical system operated at the second repetition rate.
 15. The system of claim 8, wherein the request to change the repetition rate is received from the lithography exposure apparatus.
 16. The system of claim 8, wherein the control system further comprises an operator interface, the operator interface being configured to allow operator interaction with the lithography exposure apparatus, and the request to change the repetition rate is received from the operator interface.
 17. The system of claim 8, wherein the controllable optical element is the first optical element and/or the second optical element.
 18. The system of claim 9, wherein the value of the property is determined on a pulse-by-pulse basis such that the value of the property is determined for a particular pulse and another pulse that is immediately subsequent to the particular pulse.
 19. The system of claim 18, wherein the amplified pulsed optical beam comprises at least one burst of pulses, and the value of the property is determined for fewer than all pulses in a burst of pulses.
 20. The system of claim 9, wherein the amplified pulsed optical beam comprises at least one burst of pulses, the value of the property is not determined for at least one pulse in the burst of pulses. 